Previous tire condition warning systems have operated on the principle that an under-inflated tire with a smaller rolling radius will rotate faster than a properly inflated tire travelling at the same speed. Such warning systems have compared a differential signal proportional to the difference between the rotation speeds of a pair of tires to a set reference signal which represents a particular differential signal magnitude corresponding to a particular dangerous tire condition at a particular vehicle speed.
However, such systems have a shortcoming. For example, a particular radii differential may produce a speed differential signal which exceeds the reference value at a high speed, but at a lower speed this same radii differential may produce a speed differential signal which is less than the reference value. This is because for a particular rolling radius differential the speed differential varies as the aircraft ground speed varies. As a solution to this problem, the present invention utilizes a variable reference which varies as the aircraft ground speed varies. In this manner a particular radii differential produces a speed differential signal which may attain the reference level at any aircraft ground speed.